The ability to analyse single particles, for example single cells or single beads, is useful because it allows the properties of each member of a population to have its properties determined separately. This therefore provides a greater insight than a single measurement that is simply the average of the properties of each member of the population.
A fluorescence based flow cytometry (e.g., fluorescence activated cell sorter (FACS) or the like) can measure the properties of cells or particles by scanning them as they pass through a laser beam. By labelling the cells or particles with fluorescent dyes specific to cell components, for example, receptors on the cell surface and DNA of the cell nucleus, the amount of labelled component can be detected as fluorescence when the particle or cell traverses the excitation beam. Since the amount of fluorescence emitted is proportional to the amount of fluorescent probe bound to the cell/antigen, antibodies conjugated to fluorochromes are routinely used as reagents to measure the antigen both qualitatively and quantitatively on and in the cell. Deficiencies of this approach are related to limitations and difficulties of cell staining methods and spectral overlap of fluorochromes. In other words, the detected emission of fluorochromes is not all at a specific wavelength, which means that when multiple labels are used, some of the detected emitted light can be mistakenly assigned to an incorrect label. This therefore limits the discriminatory power of the technique.
A technique, which overcomes this problem is mass cytometry [1,2]. It is analogous to flow cytometry in that a label is specifically attached to the material being analysed. The label is specifically targeted to an antigen on the cell or particle, using a specific binding partner, for example an antibody. The label is different between flow cytometry and mass cytometry. In mass cytometry, the one or more detectable labels are atoms of a known specific mass, typically transition metals, such as the rare earth metals. Accordingly, when the detectable labelling atoms are detected by the MS, it can be inferred that the target of the specific binding partner is present in the sample being analysed.